1. Field of the Invention
This invention pertains to the field of mechanical locking devices and in particular relates to certain improvements in infinitely positionable mechanical locks of the type where a rod axially displaceable through a housing is normally clamped against axial motion by coiled springs and is released by partially unwinding the springs.
23. State of the Prior Art
Infinitely positionable locking devices of the friction brake type are known in which a rod is locked with respect to the housing but where the device can be selectively actuated to an unlocked state in which such axial rod motion through the housing is enabled. One, and perhaps two, coil springs are axially fixed within a housing coaxially with the rod. The springs normally grip the curved surface of the rod to prevent the rod from moving with respect to the housing. An actuation lever permits an operator to partially unwind the springs, thereby increasing the inside diameter thereof so that the springs no longer grip the rod, which may then be moved freely axially within the coiled springs and consequently relative to the housing. In the case of a dual spring lock, two springs are disposed coaxially about a rod on either side of a central actuating lever. Each spring has a distal end engaged to one of two fixed locking bushings located at opposite ends of a housing sleeve. The interaction of the last two or three coils at the end of each spring with an angled seat surface on the corresponding bushing is responsible for the locking effect. A load tending to push the rod through the housing drives one of the springs against the angled bushing surface. As a result the last few coils of the spring are biased or canted to a locking angle against the rod. One of the springs operates to lock the rod to the housing in tension and the other locks it in compression.
Various locking devices based on principle are known, such as described in U.S. Pat. Nos. 3,874,480, 4,457,406 and 4,577,730, all owned by the assignee of this patent. Other devices of this type are described in U.S. Pat. No. 2,434,480 to Anderson issued Jan. 13, 1948, U.S. Pat. No. 3,320,590 issued to Kedem, U.S. Pat. No. 3,249,180 issued May 3, 1977 to Torossian, U.S. Pat. No. 2,429,383 to Arens, and U.S. Pat. No. 3,064,766 to Hanizesk.
These friction locks are widely used in the automotive industry for the control of reclinable seats. In early designs of these locks a phenomenon known as "walking" of the rod was observed when the device was subjected to cyclic load reversals, characterized by an incremental creeping of the rod through the lock housing. Such a cycle may consist of, for example, a seat occupant pushing back on the backrest of a seat followed by a forward force on the backrest during deceleration of the vehicle. It can also be induced by travel over rough roads. Certain improvements have largely overcome this problem for current applications of these devices. For example, in U.S. Pat. No. 4,577,730 to Porter et al., "walking" and "snap noise" are minimized or substantially eliminated by the expedient of pre-biasing the coils of the locking springs against the rod even in the absence of any load on the lock mechanism.
Nevertheless, under extreme laboratory conditions, "walking" of the rod can still be observed at cyclic load levels and frequencies substantially above those encountered in current applications of these devices. In order to extend use of these locks to ever more demanding applications, such as in very high performance vehicles, improvement is desirable.
Previous practice in friction locks has been to use springs made of smooth round wire, while the rod is given a relatively smooth surface finish by, for example, a centerless grinding process. The ground rod typically exhibits a smooth but very finely textured surface characterized by fine score lines of random length but uniformly aligned in a circumferential direction, i.e. transverse to the rod axis. Consequently, the spring surfaces gripping the rod are considerably smoother than the rod surface. In order to obtain adequate frictional locking force between the springs and the rod it has often been necessary to deliberately produce a sufficiently rough finish on the rod surface which could be gripped by the springs without slipping. While roughening of the rod surface can achieve this end, it can also cause an annoying snapping noise due to jerky engagement and disengagement of the springs and rod. This does not affect the load bearing capacity of the lock, but it is desirable to achieve smoother and quieter operation of the lock.
Ideally, the solution to these shortcomings should be adaptable to a wide variety of friction locks of the type described above with a minimum of redesign or additional components.